(1) Field of the Invention
The present invention relates to a wireless base station that communicates using an adaptive array method where transmission signals for mobile stations are spatially multiplexed using different directivity patterns.
(2) Description of the Prior Art
To raise transfer efficiency, in recent years digital communication devices have transferred information by modulating a carrier wave using a digital information signal (a baseband signal).
In the field of digital communication, frequency resources are being used ever more efficiently due to improvements in transfer speed and to increases in the number of channels that have been achieved by accommodating a plurality of users on the same frequency through time-division multiplexing. Attention is also being focused on spatial multiplexing methods where a number of channels are provided at the same time on a single frequency through the use of an adaptive array.
Adaptive array methods dynamically form directivity patterns (also called xe2x80x9carray antenna patternsxe2x80x9d) using a plurality of antennas so that radio waves only reach users positioned in certain directions. As one example, suppose that an adaptive array apparatus includes four radio units that are each composed of a transmission circuit, a reception circuit and an antenna. During transmission, the amplitude and phase of the transmission signal sent by each transmission circuit are adjusted. During reception also, the amplitude and phase of the reception signal received by each reception circuit are also adjusted. By doing so, separate directivity patterns for transmission and reception are formed. The use of an adaptive array is described in detail in Japanese in xe2x80x9cAdaptive Signal Processing in a Spatial Region and Its Applicationsxe2x80x9d in Denshi Tsushin Gakkai Ronbunshi (Transactions of the Institute of Electronics, Information, and Communication Engineers (IEICE)) Vol. J75-B-II No. 11 November), and so will not be explained here.
A wireless base station that uses an adaptive array forms different directivity patterns for each mobile station and so can simultaneously communicate with a plurality of mobile stations on a single frequency. This technique is sometimes referred to as PDMA (here standing for xe2x80x9cPath Division Multiple Accessxe2x80x9d) communication. This is described in detail in Japanese in xe2x80x9cMobile Communication using PDMA (Path Division Multiple Access)xe2x80x9d in Shingaku Giho (Communication Studies) RCS 93-84 (1994-01), pp37-44, and so will not be explained here.
A wireless base station that uses an adaptive array forms different directivity patterns to make effective use of single frequencies. However, when mobile stations that have been assigned the same frequency approach one another, each mobile station will also pick up signals that the wireless base station sends to the other mobile station. FIG. 1 shows an example where a mobile station picks up signals that the wireless base station transmits to another mobile station. In FIG. 1, the symbols xe2x80x9cPS-Axe2x80x9d to xe2x80x9cPS-Dxe2x80x9d represent mobile stations that are all assigned the same frequency. The ovals 31 to 34 drawn using solid lines represent the directivity patterns of the communication channels that are respectively directed towards the mobile stations PS-A, PS-B, PS-C, and PS-D. When the mobile station PS-B moves as shown by the arrow a, the mobile station PS-B ends up receiving signals that the wireless base station sends to the mobile station PS-A. As a result, the mobile station PS-B demodulates messages sent to the mobile station PS-A from these signals and outputs meaningless noise (similar to the noise made by a computer modem).
This is because the wireless base station performs a secret scrambling process on each signal sent to a mobile station. This scrambling process uses a unique secret key code of the mobile station to prevent other mobile stations from descrambling the signal.
As a result, each mobile station can only descramble the signals that are sent to it by the wireless base station. When a mobile station tries to descramble a signal sent to another mobile station, the secret key code of the mobile station does not match the key code that was used to scramble the signal, so that the descrambling produces a garbled, meaningless message. The mobile station outputs this message as noise. Such noise is annoying for users and so should not be outputted.
To stop mobile stations from outputting such noise, a signal frequency analyzing technique could be used to analyze signals before output by the speaker of a mobile station to detect whether the signals are for voice communication or are simply noise. When the signals are simply noise, the speaker could then be prevented from outputting the signals.
To perform this technique perfectly, however, is very difficult. There is also the problem of having to equip every mobile station with an analyzing means, which complicates the construction of mobile stations.
In view of the stated problems, it is an object of the present invention to provide a wireless base station that transmits signals in such a way that a mobile station that picks up a path-multiplexed signal intended for another mobile station does not output noise due to this signal, without the mobile station needing to be provided with any additional functions.
The stated object can be achieved by a wireless base station that uses an adaptive array antenna, including: a timing adjustment unit for adjusting a timing at which symbols are transmitted to at least one mobile station, out of a plurality of mobile stations, so that transmissions of symbols to different mobile stations are separated by a period that is shorter than one symbol period; and a multiplexing unit for spatially multiplexing symbols that are transmitted to the plurality of mobile stations using the adjusted timing, the symbols for each mobile station being transmitted using a different directivity pattern to symbols for other mobile stations.
With the stated construction, the wireless base station staggers the transmission timing of symbols for mobile stations that are path multiplexed by a predetermined amount that is less than one symbol period. As a result, when a mobile station receives a signal that is intended for another mobile station, the synch clock required to obtain the symbol data for the other signal differs from the synch clock used for the symbol data send to the mobile station. This prevents the mobile station from properly demodulating symbols that are intended for another mobile station. In this way, mobile stations can be prevented from outputting meaningless noise.
Here, the timing adjustment unit may adjust the timing for transmitting symbols to n mobile stations so that a symbol for a kth mobile station is delayed with respect to a predetermined point in a timeslot by a time equal to (k/n)xc2x7T, where k=0, 1, 2, . . . , nxe2x88x921 and T is a length of a symbol period.
With the stated construction, a wireless base station adjusts the timing at which symbols are transmitted to mobile stations located in similar directions so that there is the longest possible intervals between transmissions. This prevents mobile stations from correctly demodulating symbols intended for other mobile stations due to these symbols being transmitted with similar timing to the symbols that are intended for the mobile station.
Here, the wireless base station may further include: a multiplexed number judging unit for judging whether a number of mobile stations for which signals are spatially multiplexed is equal to or greater than a predetermined number, wherein when the number of mobile stations is equal to or above the predetermined number, the timing adjustment unit may adjust a timing at which symbols are transmitted certain mobile stations, out of the mobile stations for which signals are spatially multiplexed, so that transmissions of symbols to the certain mobile stations are separated by a period that is shorter than one symbol period, and when the number of mobile stations is below the predetermined number, the timing adjustment unit may adjust a timing at which symbols are transmitted to all the mobile stations for which signals are spatially multiplexed so that transmissions of symbols are separated by a period that is shorter than one symbol period.
When spatial multiplexing is performed for a large number of signals and the transmission timing is adjusted for every mobile station, the timing at which symbols are transmitted to different mobile stations becomes close, increasing the likelihood of a mobile station correctly demodulating symbols intended for other mobile stations. With the stated construction, however, transmission timing is only adjusted for symbols sent to certain mobile stations, thereby avoiding this problem.
Here, the wireless base station may further include: a direction vector calculating unit for calculating a direction vector from a signal received from a mobile station, the direction vector relating to a direction in which the wireless base station receives a signal from the mobile station, wherein the timing adjustment unit may select, as the certain mobile stations, mobile stations whose direction vectors, which have been calculated by the direction vector calculating means, are similar.
With the stated construction, the transmission timing is adjusted only for mobile stations which, by being positioned in similar directions relative to the wireless base station, are the most likely to mistakenly demodulate symbols intended for other mobile stations.
The stated object can also be achieved by a wireless base station that uses an adaptive array antenna, including: a direction vector calculating unit for calculating, based on a signal received from a mobile station, a direction vector showing a position of the mobile station relative to the wireless base station;a timing adjustment unit for adjusting a timing for transmitting symbols to a plurality of mobile stations that have similar direction vectors so that transmissions of symbols to different mobile stations with similar direction vectors are separated by a period that is shorter than one symbol period; and a multiplexing unit for spatially multiplexing symbols that are transmitted to the plurality of mobile stations using the adjusted timing, the symbols for each mobile station being transmitted using a different directivity pattern to symbols for other mobile stations.
With the stated construction, a wireless base station adjusts the transmission timing for a group of mobile stations that are located in similar directions with respect to the wireless base station. When a large number of mobile stations have their symbols spatially multiplexed together, a mobile station may end up correctly demodulating symbols for another mobile station using its own synch clock. However, the present wireless base station adjusts the timing at which symbols are transmitted to mobile stations that are likely to pick up symbols intended for other mobile stations, and so avoids the situation where symbols are transmitted with similar timings to mobile stations located in similar directions.